1. Field of the Invention
The present invention relates to an optical system radial deformation adjustment method and system, which corrects radial deformation that is produced in an optical system comprising at least an optical element such as an optical lens.
2. Description of the Related Art
There is an optical system that is constituted by combining at least an optical element such as an optical lens and requires high optical performances. Such an optical system minimizes deteriorations in optical performances due to manufacturing errors of the optical element and degradations in optical performances generated when manufacturing the optical element by assembling.
As an adjustment method for minimizing deteriorations in the optical performances, there are, e.g., a method for adjusting an air gap between respective optical elements by, e.g., changing a thickness of a washer between body tubes holding optical elements, a method for shifting the optical element in a vertical direction with respect to an optical axis, a method for tiling the optical element with a direction vertical to an optical axis being determined as an axis, a method for rotating the optical element around an optical axis and others as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 8-203805.
However, in the above-described adjustment methods, there remains radial deformation, which cannot be eliminated among factors that deteriorate the optical performances. This radial deformation is a phenomenon that, when light (spherical wave) from a finitely far object point existing on an optical axis of an optical system or light (plane wave) parallel to the optical axis of the optical system from infinity is transmitted through the optical system, a difference from an aplanatic ideal wavefront has a convex shape or a concave shape that can be regarded as being rotational symmetric with the optical axis of the optical system at the center on a wavefront (transmitted wavefront) of transmitted light projected from the optical system.
FIG. 13 is a view illustrating radial deformation generated in an optical element 1, and shows a model of an optical system comprising an optical element 1, which is a simple plane-parallel plate. The optical element 1, which is before radial deformation adjustment, has radial deformation. Light having a plane wave W enters a surface S of the optical element 1 in a vertical direction from an area having an even refractive index n0. The refractive index in the optical element 1 is even n. Here, the refractive index no of the area and the refractive index n in the optical element 1 have a relationship of n0<n. The light having the plane wave W that has entered the surface S of the optical element 1 in the vertical direction goes straight without being affected by refraction based on the respective refractive indices n0<n and without generating a change in the wavefront while maintaining the plane wave W.
The light having the plane wave W, after transmitted through the optical element 1, reaches the other surface Sa. Here, if a shape of the surface Sa is formed into, e.g., a convex shape that is rotational symmetric with respect to an optical axis O and pendent toward a lower side, a distance of the light moving in the optical element 1 with the refractive index n becomes long as it is close to the optical axis O and it becomes short as it is far from the optical axis O at a part having this convex shape. As a result, an optical path length of the light emitted from the other surface Sa becomes long as the light is close to the optical axis O, and advance of a wavefront Wa projected from the optical element 1 is delayed because of expansion of the optical path length. Further, the light projected from the optical element 1 is affected by refraction and become convergent light. The outgoing wavefront Wa has an upward convex shape that is rotational symmetric with respect to the optical axis O at a wavefront evaluation position H.
When the light having the plane wave W is caused to enter the optical element 1 constituted of the plane-parallel plate, an ideal wavefront of the light projected from the optical element 1 is a plane wave. The wavefront Wa with the upward convex shape that is rotational symmetric with respect to the optical axis O in the vicinity of the optical axis O in a difference between the plane wave W as the ideal wavefront and the transmitted wavefront Wa is radial deformation.
A height P of the radial deformation at the position of the optical axis O becomes n×l, which is a product of the refractive index n in the optical element 1 and a height I of the convex shape of the surface Sa at the position of the optical axis O. Since the light beam projected from the surface Sa is a convergent light beam, assuming that m is a diameter of the convex shape on the surface Sa, a diameter q of a range of the radial deformation has a relationship of q<m.
Furthermore, if the relationship of the refractive indices is n0>n or if a shape of the surface Sa is an upward convex shape that is rotational symmetric with respect to the optical axis O (recessed shape), the wavefront Wa having the convex shape in a direction opposite to the radial deformation shown in FIG. 13 becomes the radial deformation.
A main factor of such radial deformation is a surface shape including a manufacturing error of the optical element such as an optical lens constituting the optical system. The optical element such as an optical lens is manufactured by polishing a curved surface with a given curvature radius or a plane surface by using a glass material. When a portion in the vicinity of the optical axis or in the vicinity of an outer periphery is excessively or insufficiently polished in this polishing process, it is often the case that this portion is polished into a surface shape which is a convex or concave shape rotational symmetric with respect to the optical axis in a range narrower than an effective diameter (diameter of the range through which the light is transmitted) of the surface.
When the optical element having a convex or concave surface shape rotational symmetric with respect to the optical axis in the range narrower than the effective diameter (diameter of the range through which the light is transmitted) of the surface is assembled into the optical system, radial deformation occurs, and optical performances of the optical system are deteriorated. It is often the case that the radial deformation occurs to be rotational symmetric in the vicinity of the optical axis or in the vicinity of the outer periphery under the influence of the surface shape in the range narrower than the effective diameter of the surface.